As existing communication systems have become increasingly overloaded, optical transmission through transparent fibers has been found to provide a means of achieving a smaller cross-section per message, thus enabling an increased capacity within existing conduit constraints. The basic medium of transmission is an optical fiber. A first type of fiber is a stepped index fiber which comprises a transparent core member and a transparent cladding, the core member having a higher index of refraction than the cladding. Light is transmitted through the core, and contained within the core by internal reflection. So long as the light does not deviate from the fiber axis by more than the complement of the critical angle for the core-cladding interface, total internal reflection with substantially no loss results. A second type of fiber is a graded index fiber whose refractive index gradually decreases away from the fiber axis. Graded index fiber has found increasing favor due to its property that the index gradient tends to equalize the optical path length (transit time) for axial and off-axis rays. Transmission in both types of fiber is highly reliable, and is substantially insensitive to electrical noise, cross coupling between channels, and the like.
As with any communication medium, once a suitable transmission line has been found, the need arises for modules to couple sources and detectors to the line, couple lines together, perform switching, splitting, duplexing, and multiplexing functions. Ultimately, the total system can be no more reliable than these modules. When it is considered that the core of a typical optical communication fiber is characterized by a diameter of only about 50-60 microns, it can be immediately appreciated that such modules must be fabricated and installed to highly precise tolerances.
In order that the inherent reliability of optical fiber communication systems be realized, the modules themselves must be highly reliable since they are typically installed in relatively inaccessible locations (e.g. within conduits running under city streets, etc.). Given this requirement, it can be seen that it would be highly desirable to have monitoring signals that would verify the operation of the modules and the integrity of the fibers themselves. A further requirement for a satisfactory optical communication system is that the modules introduce a minimum of loss into the system. It has only been with the development of extremely high transparency fibers that optical fiber communication has become practical, and the introduction of lossy modules would considerably undercut the advantages and efficacy of such systems.
Unfortunately, existing devices for interfacing fibers to sources, detectors, and each other have proved to be lossy, bulky, delicate, and expensive. Thus, while fiber optic communication systems are proving to be highly advantageous, they are prevented from realizing their fullest potential.